Pattern forming method

ABSTRACT

According to one embodiment, an opening pattern is formed in the core film above a processing target, and a mask film is conformally formed above the processing target. Next, etch-back of the mask film is performed so that the mask film remains on a side surface of the core film. After that, line-and-space shaped core patterns, made of the core film, is formed in an area other than an area forming the opening pattern. Next, sidewall patterns are formed around the core patterns, and the core patterns are removed. Next, the processing target is patterned by using the mask film and the sidewall patterns.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2011-108532, filed on May 13, 2011; theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a pattern formingmethod.

BACKGROUND

With downsizing of semiconductor devices, it becomes difficult to form aline-and-space pattern having a width narrower than the resolution limitof lithography. In order to solve this problem, a sidewall transferringprocess has been proposed.

In the related arts, for example, a NAND type flash memory ismanufactured by using the following method which uses the sidewalltransferring process. First, in a semiconductor substrate on which atunnel insulating film, a floating gate electrode film, aninter-electrode insulating film, and a control gate electrode film arestacked, a mask film and a hard mask film are further stacked on aprocessing target film. Next, a resist pattern used to form selectiongate lines or peripheral circuits is formed on the hard mask film byusing a general photolithography technique, and the hard mask film isetched with the resist pattern as an etching mask by using an RIE(Reactive Ion Etching) method so as to form a hard mask pattern. Afterthat, in an area for forming word lines, line-and-space shaped resistpatterns having a first pitch are formed on the mask film by using thegeneral photolithography technique. After a sliming process is performedon the resist patterns, the mask film is etched with the resist patternsand the hard mask patterns as etching masks by using the RIE method soas to form mask patterns. Next, a sidewall film is conformally formed onthe processing target film with use of the mask patterns formed thereon.After performing an etch-back process, the mask patterns in the area forword lines are removed, so that closed-loop-shaped sidewall patterns areformed. Next, the processing target film is processed by using theclosed-loop-shaped sidewall patterns in the area for word lines while itis processed by using the mask pattern in the other areas. Accordingly,the word lines, the selection gate lines, and the peripheral circuits ofthe NAND type flash memory are formed.

In this manner, since an exposure process cannot be formedsimultaneously for fine patterns and relatively large-scale patternswith the photolithography technique of the related art, theline-and-space patterns for forming word lines having the smallest sizein a semiconductor device and the patterns for forming selection gatelines or peripheral circuits having relatively large sizes are formed byusing different exposure processes.

In addition, in a case where the mask is formed by using thephotolithography technique and the processing target film is processedby using the mask through the RIE method as described above, conversiondifference generally occurs in which the processed pattern becomeslarger in size than the mask, which hinders patterns from being formedwith high accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an equivalent circuit diagram illustrating a portion of amemory cell array formed in a memory cell area of a NAND type flashmemory device;

FIG. 2 is a plan view illustrating a layout pattern of a portion of thememory cell area;

FIG. 3 is a cross-sectional view taken along line A-A of FIG. 2;

FIGS. 4A to 4M are partial cross-sectional views schematicallyillustrating an example of a pattern forming method according to a firstembodiment;

FIGS. 5A to 5M are partial plan views schematically illustrating theexample of the pattern forming method according to the first embodiment;

FIGS. 6A to 6T are partial cross-sectional views schematicallyillustrating an example of a pattern forming method according to asecond embodiment;

FIGS. 7A to 7T are partial plan views schematically illustrating theexample of the pattern forming method according to the secondembodiment;

FIGS. 8A to 8U are partial cross-sectional views schematicallyillustrating an example of a pattern forming method according to a thirdembodiment;

FIGS. 9A to 9U are partial plan views schematically illustrating theexample of the pattern forming method according to the third embodiment;

FIGS. 10A to 10L are partial cross-sectional views schematicallyillustrating an example of a pattern forming method according to afourth embodiment;

FIGS. 11A to 11L are partial plan views schematically illustrating theexample of the pattern forming method according to the fourthembodiment;

FIGS. 12A to 12S are partial cross-sectional views schematicallyillustrating an example of a pattern forming method according to a fifthembodiment; and

FIGS. 13A to 13S are partial plan views schematically illustrating theexample of the pattern forming method according to the fifth embodiment.

DETAILED DESCRIPTION

In general, according to embodiments, a first core film is formed abovea processing target, an opening pattern is formed in a predeterminedarea of the first core film, and a mask film is conformally formed abovethe processing target above which the opening pattern is formed. Next,etch-back of the mask film is performed until an upper surface of thefirst core film is exposed so that the mask film remains on a sidesurface of the first core film. After that, line-and-space shaped firstcore patterns that are made of the first core film and in which widthsof line patterns have smaller than a width of the mask film remaining onthe side surface of the first core film is formed in an area other thanan area forming the opening pattern. Next, a first sidewall film isconformally formed above the processing target above which the mask filmand the first core patterns are formed, etch-back of the first sidewallfilm is performed so that upper surfaces of the first core patterns areexposed. Then, the first sidewall patterns which are made of the firstsidewall film are formed above the processing target by removing thefirst core patterns. Next, the processing target is patterned by usingthe mask film and the first sidewall patterns.

Exemplary embodiments of a pattern forming method will be explainedbelow in detail with reference to the accompanying drawings. The presentinvention is not limited to the following embodiments. In addition,cross-sectional views of a non-volatile semiconductor memory device usedfor the following embodiments are schematic views, and relation betweenthicknesses and widths of layers, ratios of thickness of layers, or thelike may be different from those of a real device. In addition, thethicknesses of layers indicated hereinafter are exemplary ones, and thepresent invention is not limited thereto.

First Embodiment

Hereinafter, the case where the embodiment is applied to a NAND typeflash memory device will be described. The NAND type flash memory deviceincludes a memory cell area where a plurality of memory cell transistors(hereinafter, referred to as memory cells) are disposed in a matrixshape and a peripheral circuit area where the peripheral circuittransistors are included to drive the memory cells.

FIG. 1 is an equivalent circuit diagram illustrating a portion of amemory cell array formed in a memory cell area of the NAND type flashmemory device. The memory cell array of the NAND type flash memorydevice is configured where NAND cell units (memory units) Su includingtwo selection gate transistors ST1 and ST2 and a memory cell row where aplurality (for example, 2^(n) (n is a positive integer)) of the memorycells MC are connected serially between the selection gate transistorsST1 and ST2 are disposed in a matrix shape. In the NAND cell unit Su, aplurality of the memory cells MC are formed such that the adjacentmemory cells share source/drain regions.

The memory cells MC which are arranged in the X direction (correspondingto a word line direction and a gate width direction) of FIG. 1 arecommonly connected by a word line (control gate line) WL. In addition,the selection gate transistors ST1 which are arranged in the X directionof FIG. 1 are commonly connected by a selection gate line SGL1, and theselection gate transistors ST2 are commonly connected by a selectiongate line SGL2. A bit line contact CB is connected to the drain regionof the selection gate transistor ST1. One end of the bit line contact CBis connected to a bit line BL which extends in the Y direction(corresponding to a bit line direction and a gate longitudinaldirection) perpendicular to the X direction of FIG. 1. In addition, theselection gate transistor ST2 is connected to a source line SL whichextends in the X direction of FIG. 1 through the source region.

FIG. 2 is a plan view illustrating a layout pattern of a portion of thememory cell area. In the semiconductor substrate 1, a plurality ofshallow trench isolation (STI) 2 are formed as element isolation areasto extend in the Y direction of FIG. 2 at a predetermined interval inthe X direction, so that, the adjacent activated areas 3 are separatedfrom each other in the X direction of FIG. 2. Word lines WL of thememory cells MC are formed to extend in the X direction of FIG. 2perpendicular to the activated area 3 at a predetermined interval in theY direction.

In addition, the two selection gate lines SGL1 which extend in the Xdirection of FIG. 2 are formed to be adjacent to each other in parallel.The bit line contacts CB are formed in the activated areas 3 between theadjacent two selection gate lines SGL1. In this example, the bit linecontacts CB are disposed such that the positions thereof in the Ydirection are alternately changed in the adjacent activated areas 3. Inother words, between the two selection gate lines SGL1, the bit linecontacts CB disposed at one selection gate line SGL1 side and the bitline contacts CB disposed at the other selection gate line SGL1 side aredisposed alternately, so-called, in a zigzag shape.

Similarly to the case of the selection gate lines SGL1, the twoselection gate lines SGL2 which extend in the X direction of FIG. 2 areformed at the positions where the selection gate lines SGL1 and apredetermined number of the word lines WL exist so as to be parallel toeach other. The source line contacts CS are disposed in the activatedareas 3 between the two selection gate lines SGL2.

The stacked gate structures MG of the memory cells MC are formed in theactivated areas 3 intersecting the word lines WL, and the gatestructures SG1 and SG2 of the selection gate transistors ST1 and ST2 areformed in the activated areas 3 intersecting the selection gate linesSGL1 and SGL2.

FIG. 3 is a cross-sectional view taken along line A-A of FIG. 2. Inother words, this figure illustrates the gate structures SG1 and SG2 ofthe selection gate transistors ST1 and ST2 and the stacked gatestructures MG of the memory cells MC disposed between the two selectiongate transistors ST1 and ST2 in the activated area 3. In FIG. 3, thestacked gate structures MG of the memory cells MC and the gatestructures SG1 and SG2 of the selection gate transistors ST1 and ST2which are formed on the semiconductor substrate 1 such as a siliconsubstrate has a structure where a floating gate electrode film 12, aninter-electrode insulating film 13, and a control gate electrode film 14are sequentially stacked through a tunnel insulating film 11. Inaddition, an opening 13 a for conduction between the floating gateelectrode film 12 and the control gate electrode film 14 is formed ineach of the inter-electrode insulating films 13 of the gate structuresSG1 and SG2 of the selection gate transistors ST1 and ST2, and a controlgate electrode film 14 is embedded in the opening 13 a. Therefore, agate electrode is configured to include the floating gate electrode film12 and the control gate electrode film 14 in each of the selection gatetransistors ST1 and ST2.

A thermal oxide film, a thermal oxide nitride film, a chemical vapordeposition (CVD) oxide film, a CVD oxide nitride film, an insulatingfilm with Si being interposed, an insulating film with Si being embeddedin a dot shape, or the like may be used as the tunnel insulating film11. A polycrystalline silicon doped with N type impurities or P typeimpurities, a metal film or a polymetal film using Mo, Ti, W, Al, Ta, orthe like, a nitride film, or the like may be used as the floating gateelectrode film 12. A silicon oxide film, a silicon nitride film, an ONO(Oxide-Nitride-Oxide) film having a stacked structure of a silicon oxidefilm and a silicon nitride film, a high dielectric film such as analuminum oxide film or a hafnium oxide film, a stacked structure of alow dielectric film such as a silicon oxide film or a silicon nitridefilm, or the like and a high dielectric film may be used as theinter-electrode insulating film 13. A polycrystalline silicon doped withN type impurities or P type impurities, a metal film or a polymetal filmusing Mo, Ti, W, Al, Ta, or the like, a stacked structure of apolycrystalline silicon film and a metal silicide film, or the like maybe used as the control gate electrode film 14.

Impurity diffusion regions 15 a which becomes source/drain regions areformed in the vicinity of the surface of the semiconductor substrate 1between the stacked gate structures MG-MG and between the stacked gatestructure MG and the gate structures SG1 and SG2. In addition, similarlyto the impurity diffusion regions 15 a, impurity diffusion regions 15 bwhich become source/drain regions are formed in the vicinity of thesurface of the semiconductor substrate 1 between the adjacent gatestructures SG1-SG1 and between the adjacent gate structures SG2-SG2.

Sidewall insulating films 16 which is made of, for example, a siliconoxide film are formed between a pair of the adjacent stacked gatestructures MG-MG, between the stacked gate structure MG and the gatestructures SG1 and SG2, on the sidewall surfaces between the gatestructures SG1-SG1, and on the sidewall surfaces between the gatestructures SG2-SG2. Herein, the sidewall insulating films 16 are formedso as to be embedded between the stacked gate structures MG-MG andbetween the stacked gate structure MG and the gate structures SG1 andSG2. However, between the gate structures SG1-SG1 and between gatestructures SG2-SG2, the sidewall insulating films 16 are not fullyembedded, but the sidewall insulating films 16 are formed to be disposedon the facing sidewall surfaces.

Impurity diffusion regions 15 c for decreasing contact resistance of thebit line contact CB and the source line contact CS are formed in thevicinity of the surface of the semiconductor substrate 1 between thesidewall insulating films 16 facing each other between the gatestructures SG1-SG1 and between the gate structures SG2-SG2. The impuritydiffusion region 15 c is formed such that the width size is narrower andthe diffusion depth (pn junction depth) is deeper than those of theimpurity diffusion region 15 b, so that an LDD (Lightly Doped Drain)structure is formed.

In addition, an interlayer insulating film 17 is formed on the stackedgate structure MG and the gate structures SG1 and SG2 where the sidewallinsulating films 16 are formed. The bit line contact CB from the uppersurface of the interlayer insulating film 17 to the surface of thesemiconductor substrate 1 is formed between the adjacent gate structuresSG1-SG1 at one end portion of a row of the memory cells MC. As describedabove, as seen in the top view, the bit line contacts CB are disposedalternately in a zigzag shape, and in the case of FIG. 3, the bit linecontact is formed at a position which is shifted rightwards. Inaddition, a source line contact CS from the upper surface of theinterlayer insulating film 17 to the surface of the semiconductorsubstrate 1 is formed across the bit lines BL between the adjacent gatestructures SG2-SG2 at the other end portion of the row of the memorycells MC.

Next, the pattern forming method is described by exemplifyingmanufacturing of a non-volatile semiconductor memory device. FIGS. 4A to4M are partial cross-sectional views schematically illustrating anexample of a pattern forming method according to a first embodiment, andFIGS. 5A to 5M are partial plan views schematically illustrating theexample of the pattern forming method according to the first embodiment.In addition, FIGS. 4A to 4M correspond to cross-sectional views takenline B-B of FIGS. 5A to 5M. In addition, these figures illustrate aportion where the two memory units Su are disposed to be adjacent toeach other in the bit line direction.

First, the tunnel insulating film 11 and the floating gate electrodefilm 12 are formed on the semiconductor substrate 1 such as apredetermined conductivity type silicon substrate, and trenches reachingthe semiconductor substrate 1 are formed by using a photolithographytechnique and an etching technique such as the RIE method. The trenchesare formed to extend in the Y direction (bit line direction) at apredetermined interval in the X direction (word line direction). Next,insulating films such as silicon oxide films are embedded in thetrenches, so that the STIs2 are formed. After that, the inter-electrodeinsulating film 13 is formed above the entire surface of thesemiconductor substrate 1, and an opening penetrating theinter-electrode insulating film 13 is formed in the area for forming theselection gate lines SGL1 and SGL2 by using the photolithographytechnique and the etching technique. Next, the control gate electrodefilm 14 is formed above the entire surface of the semiconductorsubstrate 1. In addition, although the processing objects are the tunnelinsulating film 11, the floating gate electrode film 12, theinter-electrode insulating film 13, and the control gate electrode film14 formed on the semiconductor substrate 1, in the cross-sectional viewsdescribed hereinafter, only the control gate electrode film 14 in thetop layer is illustrated and described as the processing object. Inaddition, the control gate electrode film 14 is assumed to be made ofSi.

Next, as illustrated in FIGS. 4A and 5A, a core film 31 is formed on theentire surface of the processing object (control gate electrode film14). For example, a tetraethoxysilane (TEOS) film having a thickness of200 nm may be used as the core film 31.

After that, as illustrated in FIGS. 4B and 5B, a resist (not shown) isapplied on the core film 31, and a resist pattern is formed to open anarea R including the areas for forming the selection gate transistorsST1 and ST2 between the adjacent memory units Su by using thephotolithography technique. The opening is formed such that the widththereof in the Y direction becomes a width including a pair of the gatestructures SG1 and SG2 of the selection gate transistors ST1 and ST2 andthe length thereof in the X direction becomes a length substantiallyequal to the length of the word line WL. Herein, the size in the Ydirection is set to 200 nm, and the size in the X direction is set to4000 nm. Next, the core film 31 is etched according to the RIE method byusing the resist pattern as a mask. At this time, for example, C₄F₈ orthe like of which the selection ratio to Si of the control gateelectrode film 14 can be easily taken may be used as an etching gas.Therefore, an opening 31 a is formed to extend in the X direction.

Next, as illustrated in FIGS. 4C and 5C, a mask film 32 which becomes amask for etching the selection gate lines SGL1 and SGL2 is formed on theprocessing object and the core film 31. Herein, the mask film 32 isformed such that a step difference in the opening 31 a is conformallycovered. For example, an SiN film may be used as the mask film 32. Thewidth of the mask film 32 (thickness thereof on the side surface of thecore film 31) is preferably set to the value obtained by subtracting adesired width of the word line WL (in this example, the width of twolines) from the width of the selection gate lines SGL1 and SGL2 in FIG.2, and in this example, the film is formed to have a width of 50 nm.Since a sidewall film is formed to have a width equal to that of theword line WL in the following process, the width is formed to be reducedby the amount in advance.

Subsequently, as illustrated in FIGS. 4D and 5D, etch-back of the maskfilm 32 is performed by anisotropic etching such as the RIE method untilat least the core film 31 is exposed. For example, CH₂F₂ gas or the likemay be used as a processing gas. Therefore, the mask film 32 selectivelyremains on the side surface of the core film 31, and the mask forforming the selection gate lines SGL1 and SGL2 in FIG. 2 is formed.

After that, as illustrated in FIGS. 4E and 5E, a resist 33 is applied onthe processing object, and a mask film 34 is formed thereon. Photoresist coating is further performed thereon, and resist patterns 35 isformed by a lithography technique. A core forming resist patterns 35 afor the sidewall transferring process on an area where the core film 31is formed and a resist pattern 35 b which covers the area R are formedas the resist patterns 35. In addition, in the case where etching isperformed under the condition where the control gate electrode film 14is difficult to be etched in comparison with the core film 31 in thenext etching process, the resist pattern 35 b is unnecessary. Herein,the widths of the core forming resist patterns 35 a are set to 50 nm,and the distances (spaces) between the core forming resist patterns 35 aare also set to 50 nm. In addition, herein, a multi-layer resist processstructure is formed. In this structure, thicker patterned resist isformed by transferring the resist patterns 35 to the mask film 34 onceand processing the resist 33 using the mask film 34 as a mask. However,the structure is not necessarily required.

Next, as illustrated in FIGS. 4F and 5F, the core forming resistpatterns 35 a are transferred to the core film 31 by the RIP methodusing the resist patterns 35 as a mask. At this time, a C₄F₈ based gasmay be used of which the selection ratio to the control gate electrodefilm 14 or the mask film 32 of the processing object can be easilytaken. After that, as illustrated in FIGS. 4G and 5G, the resist 33 isremoved by a resist stripping technique. The selection ratio to the corefilm 31/the mask film 32/the processing object (control gate electrodefilm 14) is taken, and the resist 33 is removed by using a gascontaining, for example, O₂ as a main component.

Next, as illustrated in FIGS. 4H and 5H, the sliming of the core films31 is performed by using the isotropic etching until the core films 31have substantially half width (herein, 25 nm). For example, wet etchingusing hydrofluoric acid may be employed as the isotropic etching. Inaddition, instead of sliming the core films 31 in FIGS. 4H and 5H, aftersliming the resist patterns 35 formed in FIGS. 4E and 5E, the slimedresist patterns 35 may be transferred to the core film 31. Sliming maybe performed on any one of films formed between the resist patterns 35and the core film 31 when the resist patterns 35 is transferred to thecore film 31, or RIE may be performed under the condition that slimingof the core films 31 themselves is performed. Alternatively, the corefilm 31 of which the width is substantially half the widths of the coreforming resist patterns 35 a may be obtained by combining the sliming ofthe core film 31 and the sliming of the resist patterns 35 or the like.

Subsequently, as illustrated in FIGS. 4I and 5I, a sidewall film 36 isformed above the entire surface of the processing object. The sidewallfilm 36 is formed to conformally cover the core films 31 formed on theprocessing object and the mask film 32. For example, an SiN film may beused as the sidewall film 36, and a thickness thereof may besubstantially equal to the width 25 nm of the core films 31.

Next, as illustrated in FIGS. 4J and 5J, etch-back of the formedsidewall film 36 is performed by anisotropic etching such as the RIEmethod until the upper surface of the core films 31 are exposed. Forexample, a CH₂F₂ gas or the like may be used as a processing gas.Therefore, loop-shaped sidewall films 36 are formed around the corefilms 31. In addition, loop-shaped sidewall films 36 are also formedaround the mask film 32, so that the size of the mask film 32 isincreased by the thickness (50 nm) of the sidewall films 36 formed inboth sides. Hereinafter, the mask film 32 on which the sidewall films 36are formed is referred to as a mask film 321.

After that, as illustrated in FIGS. 4K and 5K, the core films 31 areremoved by using an etching process by taking a selection ratio to thecontrol gate electrode film 14 and the mask film 321. Wet etching usinghydrofluoric acid may be employed as the etching process. Therefore, thesidewall films 36 formed on the side surfaces of the core films 31 areformed as new line patterns, and masks for forming the word lines WL inFIG. 2 are produced. Herein, the sizes of the word lines formation masksand the distances (spaces) between the adjacent word lines formationmasks become 25 nm, and the size of the selection gate line formationmask becomes 100 nm.

In addition, in this manner, the sidewall films 36 and the mask film 321have closed loop structures, and the end portions of a pair of theadjacent word lines WL in the X direction and the end portions of a pairof the adjacent selection gate lines SGL1 and SGL2 in the X directionare connected to each other. Next, as illustrated in FIGS. 4L and 5L, aresist is applied above the entire surface of the semiconductorsubstrate 1, and a resist pattern 37 is formed by the lithographytechnique, so that an area excluding the end portions of the word linesformation masks (sidewall films 36) in the X direction and the selectiongate line formation mask (mask film 321) is covered. Next, asillustrated in FIGS. 4M and 5M, the end portions of the word linesformation masks in the X direction and the selection gate line formationmask are removed by using an anisotropic etching process such as the RIEmethod. Therefore, word lines formation patterns 36 a which arrangedcorrectly according to the rule of the line-and-space shape andselection gate lines formation patterns 321 a of which the size islarger than that of the word lines formation patterns 36 a are formed.

After that, the processing object is etched, for example, by theanisotropic etching such as the RIE using the word lines formationpatterns 36 a and the selection gate lines formation patterns 321 a asmasks, and the word lines WL which extend in the X direction andarranged at a predetermined interval in the Y direction are formedbetween a pair of the selection gate lines SGL1 and SGL2 which extend inthe X direction.

In the first embodiment, a mask film 32 which is to be a selection gateline formation mask is formed on the core film 31 where the opening 31 ais formed such that an area where the selection gate lines SGL1 and SGL2are formed becomes a step difference. Next, a resist is applied on theprocessing object, core forming resist patterns 35 a of a sidewalltransferring process is formed, and the core film 31 is processed by theanisotropic etching process. After that, word lines formation masks areformed by using the sidewall transferring process. Herein, the size ofthe selection gate line formation mask in the Y direction is determinedby a thickness at the time of forming the mask film 32 and a thicknessat the time of forming the sidewall film 36, and the thickness of themask film 32 and the sidewall film 36 can be controlled accurately byusing a film formation technique so as to be a desired thickness.Therefore, the control of the size of the selection gate line formationmask can be accurately performed without occurrence of conversiondifference in the case where the pattern is formed by the lithographytechnique and the etching technique.

In addition, since the control of the size of the selection gate lineformation mask can be accurately performed, it is possible to obtain aneffect of reducing the area of a non-volatile semiconductor memorydevice by a conversion difference which is shown as a margin in aconventional method of processing a film by using the photolithographytechnique and the etching technique. Furthermore, it is possible toprocess the processing object without an increase in the number of workprocesses in comparison with a conventional method.

Second Embodiment

Although the case of forming the line-and-space shaped patterns of whichthe sizes (widths) are ½ of the sizes (widths) of the core patternswhich are first formed is described in the first embodiment, the case offorming line-and-space shaped patterns of which the sizes are ¼ of thesizes of core patterns which are first formed will be described in asecond embodiment.

FIGS. 6A to 6T are schematic partial cross-sectional views illustratingan example of a pattern forming method according to the secondembodiment, and FIGS. 7A to 7T are schematic partial plan viewsillustrating the example of the pattern forming method according to thesecond embodiment. In addition, FIGS. 6A to 6T correspond tocross-sectional views taken line C-C of FIGS. 7A to 7T. In addition,these figures illustrate a portion where the two memory units Su aredisposed to be adjacent to each other in the bit line direction. Inaddition, in this example, the case where the word lines WL and theselection gate lines SGL1 and SGL2 are formed by processing stackedfilms of the tunnel insulating film 11, the floating gate electrode film12, the inter-electrode insulating film 13, and the control gateelectrode film 14 formed on the semiconductor substrate 1 will bedescribed. In addition, the control gate electrode film 14 is assumed tobe made of Si.

First, as illustrated in FIGS. 6A and 7A, a mask film 51, a core film52, and a core film 53 are sequentially formed on the entire surface ofthe processing object (control gate electrode film 14). For example, aTEOS film having a thickness of 200 nm may be used as the mask film 51and the core film 53. In addition, for example, a silicon film having athickness of 50 nm may be used as the core film 52.

After that, as illustrated in FIGS. 6B and 7B, similarly to FIGS. 4B and5B of the first embodiment, an opening 53 a is formed in the core film53 of the area R including the area for forming the selection gatetransistors ST1 and ST2 between the adjacent memory units Su by thephotolithography technique and the RIE technique. The opening 53 a isformed so that the width of Y direction thereof becomes a widthincluding a pair of the gate structures SG1 and SG2 of the selectiongate transistors ST1 and ST2 and the length in the X direction thereofbecomes a length substantially equal to the length of the word line WL.Herein, the size in the Y direction is set to 200 nm, and the size inthe X direction is set to 4000 nm. In addition, for example, C₄F₈ or thelike of which the selection ratio to Si of the core film 52 can beeasily taken may be used as an etching gas.

Next, as illustrated in FIGS. 6C and 7C, a mask film 54 which becomes amask for etching the selection gate lines SGL1 and SGL2 is conformallyformed on the core films 52 and 53. For example, an SiN film may be usedas the mask film 54. The width of the mask film 54 is preferably set tothe value obtained by subtracting a desired width of the word line WLfrom the width of the selection gate lines SGL1 and SGL2 in FIG. 2, andin this example, the film is formed to have a width of 50 nm.

Subsequently, as illustrated in FIGS. 6D and 7D, etch-back of the formedmask film 54 is performed by the anisotropic etching such as the RIEmethod until at least the core film 53 is exposed. For example, CH₂F₂gas or the like may be used as a processing gas. Therefore, the mask forforming the selection gate lines SGL1 and SGL2 in FIG. 2 is formed.

After that, as illustrated in FIGS. 6E and 7E, a resist 55 is applied onthe processing object, and a mask film 56 is formed thereon. Photoresist coating is further performed thereon, and resist patterns 57 isformed by the lithography technique. The core forming resist patterns 57a for the sidewall transferring process on an area where the core film53 is formed and a resist pattern 57 b which covers the area R areformed as the resist patterns 57. In addition, in the case where etchingis performed under the condition where the core film 52 is difficult toetch in comparison with the core film 53 as the next etching process,the resist pattern 57 b is unnecessary. Herein, the widths of the coreforming resist patterns 57 a are set to 50 nm, and the distances(spaces) between the core forming resist patterns 57 a are also set to50 nm.

Next, as illustrated in FIGS. 6F and 7F, the core forming resistpatterns 57 a are transferred to the core film 53 by the RIE methodusing the resist patterns 57 as masks. At this time, a C₄F₈ based gas ofwhich the selection ratio to the core film 52 and the mask film 54 canbe easily taken may be used. After that, as illustrated in FIGS. 6G and7G, the resist 55 is removed by the resist stripping technique using agas containing, for example, O₂ as a main component.

Next, as illustrated in FIGS. 6H and 7H, the sliming is performed by theisotropic etching until the core films 53 have substantially halfwidths. For example, wet etching using hydrofluoric acid may be used asthe isotropic etching.

Subsequently, as illustrated in FIGS. 6I and 7I, a sidewall film 58 isformed above the entire surface of the processing object. The sidewallfilm 58 is formed to conformally cover the core films 53 and the maskfilm 54 formed on the core film 52. For example, an SiN film having athickness of 25 nm which is substantially equal to the width of the corefilm 53 may be used as the sidewall film 58.

Next, as illustrated in FIGS. 6J and 7J, etch-back of the formedsidewall film 58 is performed by the anisotropic etching such as the RIEmethod until the upper surfaces of the core films 53 are exposed, andloop-shaped sidewall films 58 are formed around the core films 53. Forexample, CH₂F₂ gas or the like may be used as a processing gas.Hereinafter, the mask film 54 where the sidewall films 58 are formed isdenoted by a mask film 541.

After that, as illustrated in FIGS. 6K and 7K, the core films 53 areremoved by the etching process. Wet etching using hydrofluoric acid maybe used as the etching process. Therefore, core film processing patternswhich are to be transferred to the core film 52 are formed. Herein, thewidths in the Y direction of the sidewall films 58 of the core filmprocessing patterns and the distances (spaces) between the adjacentsidewall films 58 become 25 nm, respectively.

Next, as illustrated in FIGS. 6L and 7L, the core film 52 is etched bythe etching technique such as the RIE method using the core filmprocessing patterns as masks, and the core film processing patterns aretransferred to the core film 52. For example, a gas including Cl or thelike may be used as a processing gas.

Subsequently, as illustrated in FIGS. 6M and 7M, the mask film 541 andthe sidewall films 58 are removed by etching. In the case where the corefilm 52 is made of Si, the mask film 541 and the sidewall films 58 aremade of SiN, and the mask film 51 is made of TEOS, wet etching usingphosphoric acid or the like may be used so that the mask film 541 andthe sidewall films 58 on the core film 52 take a selection ratio to thecore film 52.

After that, as illustrated in FIGS. 6N and 7N, the sliming is performedby the isotropic etching until the core films 52 in the area for formingthe word lines WL have substantially half widths. Wet etching orChemical Dry Etching (CDE) may be used as the isotropic etching. Herein,the etching is performed until the widths of the patterns formed in thearea for forming the word lines WL become 12.5 nm.

Subsequently, as illustrated in FIGS. 6O and 7O, a sidewall film 59 isconformally formed above the entire surface of the processing object.For example, an SiN film having a thickness of 12.5 nm which issubstantially equal to the widths of the core films 52 in the area forforming the word lines WL may be used as the sidewall film 59. Afterthat, as illustrated in FIGS. 6P and 7P, etch-back of the formedsidewall film 59 is performed by the anisotropic etching such as the RIEmethod until the upper surfaces of the core films 52 are exposed. Forexample, CH₂F₂ gas or the like may be used as a processing gas.Therefore, loop-shaped sidewall films 59 are formed around the corefilms 52.

Next, as illustrated in FIGS. 6Q and 7Q, a photo resist (notillustrated) is applied above the entire surface of the processingobject, and a resist pattern 60 is formed by the lithography techniqueso as to cover an area including the facing selection gate lines SGL1and SGL2 of the adjacent memory units Su.

After that, as illustrated in FIGS. 6R and 7R, the core films 52 of thearea for forming the word lines WL are removed by the etching process.Wet etching using choline may be used as the etching process. Therefore,the sidewall films 59 formed on the side surface of the core films 52are formed as new line patterns, and masks for forming the word lines WLin FIG. 2 is produced. Herein, the sizes of the word lines formationmasks and the distances (spaces) between the adjacent word linesformation masks become 12.5 nm, respectively.

Next, as illustrated in FIGS. 6S and 7S, the resist pattern 60 isremoved by the resist stripping technique using a gas containing, forexample, as a main component. Subsequently, as illustrated in FIGS. 6Tand 7T, the mask film 51 is etched by the anisotropic etching such asthe RIE method using the sidewall films 59 as masks in the area forforming the word lines WL and using the core film 52 and the sidewallfilms 59 as masks in the area for forming the selection gate lines SGL1and SGL2. Therefore, the word lines formation masks and the selectiongate line formation mask are obtained. A gas such as C₄F₈ may be used asa processing gas.

After that, as illustrated in FIGS. 4L, 4M, 5L, and 5M of the firstembodiment, the end portions of the word lines formation masks in the Xdirection and the selection gate line formation mask which constituteclosed loop structures are cut by the photolithography technique and theetching technique. Therefore, line-and-space shaped word lines formationpatterns are formed between a pair of the line-shaped selection gatelines formation patterns. Next, the stacked films from the control gateelectrode film 14 to the tunnel insulating film 11 which constitute theprocessing object are processed by the anisotropic etching such as theRIE method using the selection gate lines formation patterns and theword lines formation patterns, so that the word lines WL which arearranged at a predetermined interval in the Y direction are formed toextend in the X direction between a pair of the selection gate linesSGL1 and SGL2 which extend in the X direction.

In the second embodiment, first, the core film 52 and the core film 53are formed to overlap each other on the processing object, the opening53 a is formed by removing the core film 53 in the area where theselection gate lines SGL1 and SGL2 are formed, and the mask film 54 isconformally formed thereon. After that, etch-back is performed to form amask for forming the selection gate lines SGL1 and SGL2. Next, after thecore film 53 on the area for forming the word lines WL is processed bythe photolithography technique and the etching technique so that thewidths thereof become substantially four times of the final widths ofthe word lines, sliming is performed by the isotropic etching so thatthe widths thereof become substantially half. After that, a sidewallfilm 58 is formed, etch-back of the sidewall film 58 is performed, andthe core films 53 are removed. After core film processing patterns areformed, the core film processing patterns are transferred to the corefilm 52, and sliming is performed so that the widths of the core filmprocessing patterns become substantially half. After that, a sidewallfilm 59 is formed, and etch-back of the sidewall film 59 and removingthe core films 52 in the area for forming the word lines are performed,so that word lines formation masks and selection gate lines formationmasks are formed. In this manner, since the sizes of the selection gatelines formation masks are controlled according to the thicknesses of themask film 54 and the sidewall films 58, an amount of sliming of the corefilms 52, and the thickness of the sidewall film 59, controlling of thesizes of the selection gate lines formation masks can be accuratelyperformed in comparison with the case of forming the pattern by usingthe lithography technique and the anisotropic etching technique. Inaddition, the processing object can be processed without an increase inthe number of work processes in comparison with a conventional method.

Third Embodiment

In a third embodiment, as a method different from the second embodiment,a method of forming line-and-space shaped patterns of which the sizesare ¼ of the sizes of the core films which are first formed, will bedescribed.

FIGS. 8A to 8U are schematic partial cross-sectional views illustratingan example of a pattern forming method according to the thirdembodiment, and FIGS. 9A to 9U are schematic partial plan viewsillustrating the example of the pattern forming method according to thethird embodiment. In addition, FIGS. 8A to 8U correspond tocross-sectional views taken line D-D of FIGS. 9A to 9U. In addition,these figures illustrate a portion where the two memory units Su aredisposed to be adjacent to each other in the bit line direction. Inaddition, in this example, the case where the word lines WL and theselection gate lines SGL1 and SGL2 are formed by processing a stackedfilms of the tunnel insulating film 11, the floating gate electrode film12, the inter-electrode insulating film 13, and the control gateelectrode film 14 formed on the semiconductor substrate 1 will bedescribed. In addition, the control gate electrode film 14 is assumed tobe made of Si.

First, as illustrated in FIGS. 8A and 9A, a mask film 71 is formed onthe entire surface of the processing object (control gate electrode film14). For example, a TEOS film having a thickness of 200 nm may be usedas the mask film 71.

Next, as illustrated in FIGS. 8B and 9B, similarly to FIGS. 4B and 5B ofthe first embodiment, an opening 71 a is formed in the mask film 71 ofthe area R including the area for forming the selection gate transistorsST1 and ST2 between the adjacent memory units Su by the photolithographytechnique and the RIE technique. The opening 71 a is formed so that thewidth in the Y direction thereof becomes a width including a pair of thegate structures SG1 and SG2 of the selection gate transistors ST1 andST2 and the length in the X direction thereof becomes a lengthsubstantially equal to the length of the word line WL. Herein, the sizein the Y direction is set to 200 nm, and the size in the X direction isset to 4000 nm. In addition, for example, C₄F₈ or the like of which theselection ratio to silicon of the control gate electrode film 14 can beeasily taken may be used as an etching gas.

After that, as illustrated in FIGS. 8C and 9C, a mask film 72 whichbecomes a mask for etching the selection gate lines SGL1 and SGL2 isconformally formed on the control gate electrode film 14 and the maskfilm 71. For example, an SiN film may be used as the mask film 72. Thewidth of the mask film 72 is preferably set to the same value as thewidth of the selection gate lines SGL1 and SGL2 in FIG. 2, and in thisexample, the film is formed to have a width of 50 nm.

Subsequently, as illustrated in FIGS. 8D and 9D, etch-back of the formedmask film 72 is performed by the anisotropic etching such as the RIEmethod until at least the mask film 71 is exposed. For example, CH₂F₂gas or the like may be used as a processing gas. Therefore, the mask forforming the selection gate lines SGL1 and SGL2 in FIG. 2 is formed.

After that, as illustrated in FIGS. 8E and 9E, a mask film 73 is formedabove the entire surface of the processing object so that the internalportion of the closed-loop-shaped mask film 72 is embedded. For example,a TEOS film having a thickness of 200 nm may be used as the mask film73. Hereinafter, the mask films 71 and 73 made of the same material arecollectively referred to as a mask film 711.

As illustrated in FIGS. 8F and 9F, the upper surface of the mask film711 is planarized according to a Chemical Mechanical Polishing (CMP)method. At this time, the planarization is performed by using the maskfilm 72 as a stopper.

After that, as illustrated in FIGS. 8G and 9G, a core film 74 is formedabove the entire surface of the processing object. For example, asilicon film having a thickness of 50 nm may be used as the core film74. In addition, as illustrated in FIGS. 8H and 9H, a core film 75 isformed on the core film 74. For example, a TEOS film having a thicknessof 200 nm may be used as the core film 75.

Next, as illustrated in FIGS. 8I and 9I, a resist 76 is applied abovethe processing object, and a mask film 77 is formed thereon. A photoresist is further applied thereon, and a core forming resist patterns 78for the sidewall transferring process are formed on the area for formingthe word lines WL by the lithography technique. Herein, the widths ofthe core forming resist patterns 78 are set to 50 nm, and the distances(spaces) between the core forming resist patterns 78 are also set to 50nm.

After that, as illustrated in FIGS. 8J and 9J, the resist patterns 78are transferred to the core film 75 by the RIE method. At this time, theetching is performed under the condition where the selection ratio tothe core film 74 can be easily taken. Herein, since the core film 74 ismade of Si and the core film 75 is made of TEOS, a C₄F₈ based gas ofwhich the selection ratio to Si can be taken may be used. After that,the resist 76 is removed by the resist stripping technique using a gascontaining, for example, O₂ as a main component.

Next, as illustrated in FIGS. 8K and 9K, the sliming is performed by theisotropic etching until the core films 75 have substantially halfwidths. For example, wet etching using hydrofluoric acid may be used asthe isotropic etching. Therefore, the core films 75 have widths of 25nm.

Subsequently, as illustrated in FIGS. 8L and 9L, a sidewall film 79 isformed above the entire surface of the processing object. The sidewallfilm 79 is formed so that the core films 75 formed on the core film 74is conformally covered. For example, an SiN film having a thickness of25 nm which is substantially equal to the widths of the core films 75may be used as the sidewall film 79.

Next, as illustrated in FIGS. 8M and 9M, etch-back of the formedsidewall film 79 is performed by the anisotropic etching such as the RIEmethod until the upper surfaces of the core films 75 are exposed, andloop-shaped sidewall films 79 are formed around the core films 75. Forexample, CH₂F₂ gas or the like may be used as a processing gas.

After that, as illustrated in FIGS. 8N and 9N, the core films 75 areremoved by the etching process. Wet etching using hydrofluoric acid maybe used as the etching process. Therefore, core film processing patternswhich are to be transferred to the core film 74 are formed. Herein, thecore film processing patterns have sizes which are ½ of the sizes of thecore forming resist patterns 78 formed by using the photolithographyprocess, and the widths of the sidewall films 79 which are the coreprocessing patterns and the distances (spaces) between the adjacentsidewall films 79 become 25 nm.

In addition, as illustrated in FIGS. 8O and 9O, the core film processingpatterns which are made of the sidewall films 79 are transferred to thecore film 74 according to the RIE method. Subsequently, as illustratedin FIGS. 8P and 9P, the sidewall films 79 constituting the core filmprocessing patterns are removed by wet etching. At this time, since thesidewall films 79 and the mask film 72 which becomes the selection gateline formation mask pattern are made of the same material (SiN), theetching time is adjusted to a degree that the mask film 72 is notcompletely removed. For example, wet etching using a solution containingphosphoric acid may be exemplified as the above wet etching.

After that, as illustrated in FIGS. 8Q and 9Q, the sliming is performedby the isotropic etching until the core films 74 in the area for formingthe word lines WL have substantially half widths. Wet etching or CDE maybe used as the isotropic etching. Herein, the etching is performed untilthe widths of the core film processing patterns formed in the area forforming the word lines WL becomes 12.5 nm.

Subsequently, as illustrated in FIGS. 8R and 9R, a sidewall film 80 isconformally formed above the entire surface of the processing object.For example, an SiN film having a width of 12.5 nm which issubstantially equal to the width of the core film 74 may be used as thesidewall film 80. After that, as illustrated in FIGS. 8S and 9S,etch-back of the formed sidewall film 80 is performed by the anisotropicetching such as the RIE method until the upper surfaces of the corefilms 74 are exposed. For example, CH₂F₂ gas or the like may be used asa processing gas. Therefore, loop-shaped sidewall films 80 are formedaround the core films 74. Hereinafter, a combination of the mask film 72and the sidewall film 80 is denoted by a mask film 721.

After that, as illustrated in FIGS. 8T and 9T, the core films 74 areremoved by the etching process. Wet etching using choline may be used asthe etching process. Therefore, the sidewall films 80 formed on the sidesurface of the core films 74 are formed as new line patterns, and masksfor forming the word lines WL in FIG. 2 are produced. Herein, the sizesof the word lines formation masks and the distances (spaces) between theadjacent word lines formation masks become 12.5 nm. In addition, thesizes of the selection gate lines formation mask film 721 becomes 50 nm,respectively.

Next, as illustrated in FIGS. 8U and 9U, the mask film 711 is etched bythe anisotropic etching such as the RIE method using the sidewall films80 as masks in the area for forming the word lines WL. A gas such asC₄F₈ may be used as a processing gas. Therefore, the selection gatelines formation mask is formed by the mask film 721, and the word linesformation masks is formed by using the mask film 711 and the sidewallfilms 80.

After that, as illustrated in FIGS. 4L, 4M, 5L, and 5M of the firstembodiment, the end portions of the word lines formation masks in the Xdirection and the selection gate line formation mask which constituteclosed loop structures are cut by the photolithography technique and theetching technique. Therefore, line-and-space shaped word lines formationpatterns are formed between a pair of the line-shaped selection gatelines formation patterns. Next, the stacked films from the control gateelectrode film 14 to the tunnel insulating film 11 which constitute theprocessing object are processed by the anisotropic etching such as theRIE method using the selection gate lines formation patterns and theword lines formation patterns, so that the word lines WL which arearranged at a predetermined interval in the Y direction are formed toextend in the X direction between a pair of the selection gate linesSGL1 and SGL2 which extend in the X direction.

In the third embodiment, after the selection gate line formation patternis first formed, the core films 74 and 75 are formed thereon to embedthe selection gate lines formation pattern, and the word lines formationpattern is formed. Therefore, the sidewall films 79 and 80 which areformed at the time of forming the word lines formation pattern are notformed around the selection gate lines formation pattern. As a result,the sizes of the selection gate lines formation pattern does not dependon the thicknesses of the sidewall films 79 and 80. Therefore, as anadditional effect of the second embodiment, the sizes of the selectiongate lines formation pattern can be easily controlled compare with thesecond embodiment.

Fourth Embodiment

In a fourth embodiment, as a method different from those of the first tothird embodiments, a pattern forming method capable of accuratelyforming a size of a selection gate line of which the size is larger thanthe word line will be described.

FIGS. 10A to 10L are schematic partial cross-sectional viewsillustrating an example of a pattern forming method according to thefourth embodiment, and FIGS. 11A to 11L are schematic partial plan viewsillustrating the example of the pattern forming method according to thefourth embodiment. In addition, FIGS. 10A to 10L correspond tocross-sectional views taken line E-E of FIGS. 11A to 11L. In addition,these figures illustrate a portion where the two memory units Su aredisposed to be adjacent to each other in the bit line direction. Inaddition, in this example, the case where the word lines WL and theselection gate lines SGL1 and SGL2 (hereinafter, in this embodiment,simply indicated by SGL) are formed by processing stacked films of thetunnel insulating film 11, the floating gate electrode film 12, theinter-electrode insulating film 13, and the control gate electrode film14 formed on the semiconductor substrate 1 will be described. Inaddition, the control gate electrode film 14 is assumed to be made ofSi.

First, as illustrated in FIGS. 10A and 11A, a mask film 91, a stopperfilm 92, and a core film 93 are sequentially formed on the entiresurface of the processing object (control gate electrode film 14). Forexample, a TEOS film having a thickness of 150 nm may be used as themask film 91. For example, a silicon film having a thickness of 30 nmmay be used as the stopper film 92. For example, a TEOS film having athickness of 100 nm may be used as the core film 93. In addition, aresist is applied on the core film 93 through an anti-reflection film94, and resist patterns 95 are formed by the lithography technique. Asthe resist patterns 95, the line-and-space shaped core forming patterns95 a which extends in the X direction is formed in the area for formingthe word line WL, and the pattern 95 b is formed in the area for formingthe two facing selection gate lines SGL of the memory units Su which areadjacent to each other in the Y direction so as to cover the area.Herein, the widths of the line in the Y direction and the spaces of theline-and-space shaped patterns 95 a are set to 50 nm, and the width ofthe pattern 95 b in the Y direction is set to 300 nm. In addition, theline-and-space shaped patterns 95 a are connected to the end portion inthe X direction between a pair of the adjacent patterns 95 a, so thatloop shapes are formed.

Next, as illustrated in FIGS. 10B and 11B, the core film 93 is etched bythe RIE method using the resist patterns 95 as masks, and the patterns95 a and 95 b are transferred to the core film 93. At this time, theprocess is performed under the condition that the selection ratio to thestopper film 92 is high so that the stopper film 92 is used as astopper. For example, a gas such as C₄F₈/Ar/O₂ of which the selectionratio to the stopper film 92 can be easily taken may be used as aprocessing gas.

After the resist patterns 95 are transferred to the core film 93,sliming is performed until the widths of the core films 93 in the Ydirection becomes a substantially half widths, that is, 25 nm. Thesliming process may be performed according to the RIE method subsequentto the transferring of the resist patterns 95 to the core film 93.However, in the case where it is difficult to perform the sliming byonly the RIE method, the widths of the line-and-space shaped patterns 95a may be reduced in advance by the lithography technique, or the slimingmay be performed on the core films 93 where the patterns are formed bythe wet etching or the isotropic etching such as CDE. Therefore, theline-and-space shaped core patterns 93 a are formed in the area forforming the word lines WL, and the core pattern 93 b is formed in thearea for forming the selection gate lines SGL so as to cover the area.In addition, the width of the line of the core pattern 93 a in the Ydirection becomes 25 nm, and the width of the space thereof in the Ydirection becomes 75 nm.

After that, as illustrated in FIGS. 10C and 11C, a sidewall film 96 isformed above the entire surface of the processing object. The sidewallfilm 96 is formed so as to conformally cover the core patterns 93 a and93 b formed on the stopper film 92. For example, an SiN film having awidth of 25 nm which is substantially equal to the widths of the corepatterns 93 a may be used as the sidewall film 96.

Next, as illustrated in FIGS. 10D and 11D, etch-back of the formedsidewall film 96 is performed by the anisotropic etching such as the RIEmethod until the upper surfaces of the core patterns 93 a and 93 b areexposed. Herein, the process is performed under the condition where theselection ratio to the core patterns 93 a and 93 b and the stopper film92 is high. For example, CH₂F₂/Ar or the like may be used as aprocessing gas.

After that, as illustrated in FIGS. 10E and 11E, the core patterns 93 aand 93 b are removed by the etching process. Wet etching usinghydrofluoric acid may be used as the etching process. Therefore,line-and-space shaped sidewall patterns 96 a of which the line width andthe space width are 25 nm is formed in the area for forming the wordlines WL, and a sidewall pattern 96 b is formed so as to surround thearea for forming the adjacent two selection gate lines SGL. In addition,the sidewall patterns 96 a and 96 b are connected to other sidewallpatterns 96 a and 96 b through the end portions in the X direction, sothat closed loop structures are formed.

Next, as illustrated in FIGS. 10F and 11F, a mask film 97 which becomesa mask for etching the selection gate lines SGL is formed on the stopperfilm 92 where the sidewall patterns 96 a and 96 b are formed. At thistime, the mask film 97 is formed so that a portion between the adjacentsidewall patterns 96 a and a portion between the sidewall pattern 96 aand the sidewall pattern 96 b are embedded and so that a film having athickness required for processing the selection gate line SGL is formedin an inner side surface of the closed-loop-shaped sidewall pattern 96b. For example, Si with a thickness of 75 nm is formed as the mask film97. In addition, herein, although not shown, in the case where a patternof a peripheral circuit or the like is formed in an area excluding thearea for forming the word lines WL, after that, a resist pattern forcovering the mask film 97 is formed by the lithography technique.

After that, as illustrated in FIGS. 10G and 11G, etch-back of the formedmask film 97 is performed by the anisotropic etching such as the RIEmethod until the upper surfaces of the sidewall patterns 96 a and 96 bare exposed. Herein, the process is performed under the condition wherethe selection ratio to the mask film 91 is high. For example, Cl₂ or thelike may be used as the processing gas. In addition, since the stopperfilm 92 is also made of Si, it is preferable that the end point, wherethe full-surface etch-back is ended so that too much etching is notperformed, be defined in advance by using a light emitting monitor orthe like.

Next, as illustrated in FIGS. 10H and 11H, the sidewall patterns 96 aand 96 b are removed by the etching process. Wet etching usingphosphoric acid or the like may be used for removing the sidewallpatterns 96 a and 96 b. Therefore, line-and-space shaped mask patterns97 a and a dummy pattern 97 d are formed in the area for forming theword lines WL, and a mask pattern 97 b of which the width is larger thanthat of the mask pattern 97 a is formed in the area for forming theselection gate lines SGL (the end portion of the area in the Y directionfor forming the word lines WL).

After that, as illustrated in FIGS. 10I and 11I, etch-back of the entiresurface of the stopper film 92 is performed by the anisotropic etchingsuch as the RIE method. Herein, the process is performed under thecondition where the selection ratio to the mask film 91 is high. Forexample, Cl₂/O₂ or the like may be used as the processing gas.Therefore, patterns are formed which process the mask film 91 where thestopper film 92 and the mask film 97 are stacked. In addition, althougheach pattern is made of a stacked structure of the stopper film 92 andthe mask film 97, herein, the patterns made of the stacked structure areassumed to be mask patterns 97 a, 97 b, and 97 d.

Next, as illustrated in FIGS. 10J and 11J, the mask film 91 is processedby the anisotropic etching such as the RIE method using the maskpatterns 97 a, 97 b, and 97 d, which are obtained in FIGS. 10I and 11I,as masks. Herein, the process is performed under the condition where theselection ratio to the mask patterns 97 a, 97 b, and 97 d is high. Forexample, C₄F₈/Ar/O₂ or the like may be used as the processing gas.Therefore, mask patterns 91 a for forming the word lines WL, a maskpattern 91 b for forming the selection gate lines SGL, and a maskpattern 91 d for forming the dummy patterns are formed in the mask film91.

After that, as illustrated in FIGS. 10K and 11K, the processing objectis etched by the anisotropic etching such as the RIE method using themask patterns 91 a, 91 b, and 91 d as masks. Therefore, a groupincluding a predetermined number of the word lines WL which extend inthe X direction and arranged at a predetermined interval in the Ydirection and a pair of the dummy patterns DM and a pair of theselection gate lines SGL, which are disposed at both ends of the groupof the word lines WL in the Y direction and extend in the X direction,are formed. In addition, FIG. 11K illustrates the state where the STI2as the base is exposed by etching the processing object at the endportions of the selection gate lines SGL in the X direction and the wordlines WL.

In this manner, the selection gate lines SGL, the word lines WL, and thedummy patterns DM constitute closed loop structures. In other words, theend portions of the selection gate lines SGL, the word lines WL, and thedummy patterns DM in the X direction are connected to the otherselection gate lines SGL, the other word lines WL, and the other thedummy patterns DM. A resist is applied above the processing object. Sothat the connection portions of the end portions of the selection gatelines SGL, the word line WL, and the dummy pattern DM in the X directionare exposed, and other portions are covered by the lithographytechnique, patterning is performed. Therefore, a resist pattern 98 isformed.

Next, as illustrated in FIGS. 10L and 11L, the processing object isetched by the anisotropic etching such as the RIE method using theresist pattern 98 as a mask, so that closed loops cutting process isperformed. Therefore, the connection portions of the end portions of theselection gate lines SGL, the word lines WL, and the dummy patterns DMin the X direction in the area which is not covered by the resistpattern 98 are removed. After that, the resist pattern 98 is removed byusing the resist stripping technique. Accordingly, line-and-space shapedword lines WL, which extend in the X direction through the dummypatterns DM and are disposed at a predetermined interval in the Ydirection, are formed between a pair of the selection gate lines SGLwhich extend in the X direction.

In the fourth embodiment, a core pattern 93 b which covers the area forforming the two selection gate lines SGL disposed to be adjacent to eachother in the Y direction and line-and-space shaped core patterns 93 awhich are disposed at the both sides of the core pattern 93 b in the Ydirection are formed, and after sidewall patterns 96 a and 96 b areformed on the side surfaces of the core patterns 93 a and 93 b, the corepatterns 93 a and 93 b are removed. Next, a mask film 97 is formed, andetch-back of the mask film 97 is performed, and after that, the sidewallpatterns 96 a and 96 b are removed. Accordingly, the mask pattern 97 bcorresponding to the selection gate lines SGL and the mask patterns 97 acorresponding to the word lines WL are formed. Herein, although the sizeof the mask pattern 97 b in the Y direction corresponding to theselection gate line SGL is determined according to the thickness at thetime of forming the mask film 97, the thickness of the mask film 97 canbe accurately controlled by using a film forming technique so as to be adesired thickness. Therefore, unlike the case of forming a pattern byusing the lithography technique and the etching technique, conversiondifference does not occur, so it is possible to accurately control thesize of the mask pattern 91 b for forming the selection gate lines SGL.In addition, in the fourth embodiment, the area for forming the twoselection gate lines SGL may not covered with the pattern 95 b, and thecore pattern 93 b may not be required.

Although the core pattern 93 b is not provided, since the mask film 97can be formed on the side surface of the sidewall pattern 96 b withoutembedding between the adjacent sidewall patterns 96 a at the endportions of the line-and-space shaped sidewall patterns 96 a in the Ydirection formed in the area for forming the word lines WL, the maskpattern 97 b having a large width can be obtained in the area forforming the selection gate lines SGL by performing etch-back of the maskfilm 97 similarly to the case of forming the core pattern 93 b. Inaddition, in this case, if the patterns 95 a having larger widths thanothers are formed at the end portions in the Y direction as the resistpatterns 95, as a result, the mask film 97 which is formed to have alarger width than others is embedded between the adjacent sidewallpatterns 96 can be formed at the end portions in the Y direction as themask pattern 97 d having a large width corresponding to the dummypattern.

In addition, unlike the first to third embodiments, since the maskpattern 97 b corresponding to the selection gate lines SGL and the maskpatterns 97 a corresponding to the word lines WL are simultaneouslyformed, it is possible to obtain an effect of keeping the distances(spaces) between the selection gate line SGL and the word line WL whichis closest to the selection gate line SGL to be a predetermined value.

Fifth Embodiment

Although the case of forming the line-and-space shaped patterns of whichthe sizes (widths) are ½ of the sizes (widths) of the core patternswhich are first formed is described in the fourth embodiment, the caseof forming line-and-space shaped patterns of which the sizes are ¼ ofthe sizes of core patterns which are first formed will be described in afifth embodiment.

FIGS. 12A to 12S are schematic partial cross-sectional viewsillustrating an example of a pattern forming method according to thefifth embodiment, and FIGS. 13A to 13S are schematic partial plan viewsillustrating the example of the pattern forming method according to thefifth embodiment. In addition, FIGS. 12A to 12S correspond tocross-sectional views taken line F-F of FIGS. 13A to 13S. In addition,these figures illustrate a portion where the two memory units Su aredisposed to be adjacent to each other in the bit line direction. Inaddition, in this example, the case where the word line WL and theselection gate lines SGL1 and SGL2 (hereinafter, in this embodiment,simply indicated by SGL) are formed by processing a stacked films of atunnel insulating film 11, a floating gate electrode film 12, aninter-electrode insulating film 13, and a control gate electrode film 14formed on the semiconductor substrate 1 will be described. In addition,the control gate electrode film 14 is assumed to be made of Si.

First, as illustrated in FIGS. 12A and 13A, a mask film 111, a stopperfilm 112, and a core film 113 are sequentially formed on the entiresurface of the processing object (control gate electrode film 14). Forexample, a TEOS film having a thickness of 150 nm may be used as themask film 111. For example, a silicon film having a thickness of 30 nmmay be used as the stopper film 112. For example, a carbon film having athickness of 100 nm may be used as the core film 113. In addition, aresist is applied on the core film 113 through a mask film 114 made of aspin on glass (SOG) film or the like, and resist patterns 115 are formedby the lithography technique. As the resist pattern 115, theline-and-space shaped core forming patterns 115 a which extends in the Xdirection is formed in the area where the word lines WL is formed, andthe pattern 115 b is formed in the area for forming the two facingselection gate lines SGL of the memory units Su which are adjacent toeach other in the Y direction so as to cover the area. Herein, thewidths of the line in the Y direction and the space of theline-and-space shaped patterns 115 a are set to 60 nm, and the width ofthe pattern 115 b in the Y direction is set to 300 nm.

Next, as illustrated in FIGS. 12B and 13B, the resist patterns 115 aretransferred to the mask film 114 by the RIE method, and the pattern istransferred to the core film 113. At this time, the process is performedunder the condition that the selection ratio to the stopper film 112 ishigh so that the stopper film 112 is used as a stopper. For example, agas such as O₂/CH₄ of which the selection ratio to the stopper film 112can be easily taken may be used as a processing gas.

After the resist patterns 115 is transferred to the core film 113,sliming is performed until the widths of the core films 113 in the Ydirection become a substantially half widths, that is, 30 nm. Thesliming process may be performed according to the RIE method subsequentto the transferring of the resist patterns 115 to the core film 113.However, in the case where it is difficult to perform the sliming byusing only the RIE method, the widths of the line-and-space shapedpatterns 115 a may be reduced in advance by using the lithographytechnique. Therefore, the line-and-space shaped core patterns 113 a areformed in the area for forming the word lines WL, and the core pattern113 b is formed in the area for forming the selection gate lines SGL soas to cover the area. In addition, the widths of the line of the corepatterns 113 a in Y direction become 30 nm, and the widths of the spacesthereof in the Y direction become 90 nm. In addition, similarly to thefourth embodiment, the pattern 115 b illustrated in FIGS. 12A and 13Awhich covers the area for forming the two selection gate lines SGL maynot be formed, and herein, the core pattern 113 b may not be formed inthe area for forming the selection gate lines SGL.

After that, as illustrated in FIGS. 12C and 13C, a sidewall film 116 isformed above the entire surface of the processing object. The sidewallfilm 116 is formed so as to conformally cover the core patterns 113 aand 113 b formed on the stopper film 112. For example, an SiN filmhaving a thickness of 30 nm which is substantially equal to the widthsof the core patterns 113 a may be used as the sidewall film 116.

Next, as illustrated in FIGS. 12D and 13D, etch-back is performed on theformed sidewall film 116 by the anisotropic etching such as the RIEmethod until the upper surfaces of the core patterns 113 a and 113 b areexposed. Herein, the process is performed under the condition where theselection ratio to the core patterns 113 a and 113 b and the stopperfilm 112 is high. For example, CH₂F₂/Ar or the like may be used as aprocessing gas.

After that, as illustrated in FIGS. 12E and 13E, the core patterns 113 aand 113 b are removed. Herein, since carbon films are used as the corepatterns 113 a and 113 b, a stripping technique using O₂ plasma may beused. Therefore, line-and-space shaped sidewall patterns 116 a of whichthe line width and the space width are 30 nm are formed in the area forforming the word lines WL, and a sidewall pattern 116 b is formed so asto surround the area for forming the adjacent two selection gate linesSGL. In addition, the sidewall patterns 116 a and 116 b are connected toother sidewall patterns 116 a and 116 b through the end portions in theX direction, so that closed loop structures are formed. In addition,although the sidewall patterns 116 a are disposed at a substantiallyequal interval in the Y direction, the distances (spaces) between thesidewall pattern 116 b and the sidewall pattern 116 a adjacent to thesidewall pattern 116 b is configured to be larger than the intervalbetween the sidewall patterns 116 a.

Next, as illustrated in FIGS. 12F and 13F, a core film 117 which becomesa mask for etching the selection gate lines SGL and becomes a core forforming the word lines WL is formed on the stopper film 112 where thesidewall patterns 116 a and 116 b are formed. At this time, the corefilm 117 is formed so that a portion between the adjacent sidewallpatterns 116 a and a portion between the sidewall pattern 116 a and thesidewall pattern 116 b are embedded and so that a film having athickness required for processing the selection gate lines SGL is formedin an inner side surface of the closed-loop-shaped sidewall pattern 116b. For example, Si with a thickness of 75 nm is formed as the core film117. In addition, herein, although not shown, in the case where apattern of a peripheral circuit or the like is formed in an areaexcluding the area for forming the word lines WL, after that, a resistpattern for covering the core film 117 is formed by using thelithography technique.

After that, as illustrated in FIGS. 12G and 13G, etch-back of the formedcore film 117 is performed by the anisotropic etching such as the RIEmethod until the upper surfaces of the sidewall patterns 116 a and 116 bare exposed. Herein, the process is performed under the condition wherethe selection ratio to the mask film 111 is high. For example, Cl₂ orthe like may be used as the processing gas. In addition, since thestopper film 112 is also made of Si, it is preferable that the endpoint, where the etch-back of the entire surface is ended so that toomuch etching is not performed, be defined in advance by using a lightemitting monitor or the like.

Next, as illustrated in FIGS. 12H and 13H, the sidewall patterns 116 aand 116 b are removed by the etching process. Wet etching usingphosphoric acid or the like may be used for removing the sidewallpatterns 116 a and 116 b. After that, as illustrated in FIGS. 12I and13I, etch-back of the entire surface of the stopper film 112 isperformed by the anisotropic etching such as the RIE method. Herein, theprocess is performed under the condition where the selection ratio tothe mask film 111 is high. For example, Cl₂/O₂ or the like may be usedas a processing gas. Therefore, core patterns can be obtained.Hereinafter, the pattern where the stopper films 112 and the core films117 are stacked is denoted by core patterns 1171. More specifically,line-and-space shaped core patterns 1171 a, a pair of dummy corepatterns 1171 d disposed at the end portion of a group in the Ydirection including a plurality of the core patterns 1171 a, and a corepattern 1171 b for forming the selection gate lines SGL are formed.

Next, as illustrated in FIGS. 12J and 13J, sliming of the core pattern1171 is performed by the isotropic etching until the widths of the corepatterns 1171 a in the Y direction become a substantially half widths,that is, 15 nm. For example, CDE may be sued as the isotropic etching.Therefore, the width of the core pattern 1171 b in the Y directionbecomes 55 nm.

After that, as illustrated in FIGS. 12K and 13K, a sidewall film 118 isformed above the entire surface of the processing object. The sidewallfilm 118 is formed so as to conformally cover the pattern of the corepatterns 1171 formed on the mask film 111. For example, an SiN filmhaving a thickness of 15 nm which is substantially equal to the widthsof the core patterns 1171 a after sliming may be used as the sidewallfilm 118.

Next, as illustrated in FIGS. 12L and 13L, etch-back of the formedsidewall film 118 is performed by the anisotropic etching such as theRIE method until the upper surfaces of the core patterns 1171 a, 1171 b,and 1171 d are exposed. Herein, the process is performed on the corepatterns 1171 a, 1171 b, and 1171 d and the mask film 111 under thecondition of a high selection ratio. For example, CH₂F₂/Ar or the likemay be used as a processing gas.

After that, as illustrated in FIGS. 12M and 13M, a resist is appliedabove the entire surface of the processing object, and patterning isperformed by the lithography technique so that an area including a pairof the selection gate lines SGL, which are adjacent to each other in theY direction, is covered and the other areas are exposed, so that aresist pattern 119 is formed. Next, as illustrated in FIGS. 12N and 13N,the core patterns 1171 a and 1171 d are removed by the etching process.Wet etching using choline may be used as the etching process. Inaddition, as illustrated in FIGS. 12O and 13O, the resist pattern 119 isremoved by the resist stripping technique using 0 plasma. Therefore,sidewall patterns 118 a which become patterns for forming the word linesWL and a dummy pattern 118 d are formed in the area for forming the wordlines WL, and a pattern 120 for forming the selection gate lines SGLwhich is made of the core pattern 1171 b and sidewall patterns 118 bformed to surround the side surface thereof is formed in the area forforming the selection gate lines SGL.

Next, as illustrated in FIGS. 12P and 13P, the mask film 111 isprocessed by the anisotropic etching such as the RIE method using thepattern 120, the sidewall patterns 118 a, and the dummy pattern 118 d,which are obtained by the processes up to the process of FIGS. 12O and13O, as a mask.

Herein, the process is performed under the condition where the selectionratio to the processing object is high. For example, a gas such asC₄F₈/Ar/O₂ may be used as the processing gas. Therefore, mask patterns111 a for forming the line-and-space shaped word lines WL of which theline width and the space width are 15 nm and mask patterns 111 d forforming the dummy pattern of which the line width is 15 nm and which islocated at the both ends of a group in the Y direction including apredetermined number of the mask patterns 111 a for forming the wordlines WL are formed in the area for forming the word lines WL, and maskpattern 111 b for forming the selection gate lines SGL of which thewidth in the Y direction is 85 nm are formed in the area for forming theselection gate lines SGL.

After that, as illustrated in FIGS. 12Q and 13Q, the processing objectis etched by the anisotropic etching such as the RIE method using themask patterns 111 a, 111 b, and 111 d as a mask. Therefore, the wordlines WL which are arranged at a predetermined interval in the Ydirection and the dummy patterns DM are formed to extend in the Xdirection between a pair of the selection gate lines SGL which extend inthe X direction. In addition, FIG. 13Q illustrates the state where thesemiconductor substrate (silicon substrate) 1 which constitutes STI2 asthe base is exposed by etching the processing object. In addition, thetwo dummy patterns DM are disposed between the selection gate line SGLand the word line WL. Unlike the word lines WL, since these patterns arenot arranged at a predetermined interval in the Y direction, thesepatterns become the dummy patterns which are not used as wire lines forthe memory cell.

In this manner, the patterned processing objects (the selection gateline SGL, the word lines WL, and the dummy pattern DM) constitute closedloop structures. In other words, the end portions of the selection gatelines SGL, the word lines WL, and the dummy patterns DM in the Xdirection are connected to the other selection gate lines SGL, the otherword lines WL, and the other dummy patterns DM. Therefore, a process forremoving the connection portions of the end portions in the X directionis performed. Herein, as illustrated in FIGS. 12R and 13R, a resist isapplied on the processing object, the connection portions of the endportions of the selection gate lines SGL and the word lines WL in the Xdirection are exposed by the lithography technique, and patterning isperformed so as to cover other portions, so that a resist pattern 121 isformed.

Next, as illustrated in FIGS. 12S and 13S, the processing object isetched by the anisotropic etching such as the RIE method using theresist pattern 121 as a mask, so that closed loops cutting process isperformed. Herein, the selection gate line SGL, the word line WL, andthe dummy pattern DM in the area which is not covered by the resistpattern 121 are removed. After that, the resist pattern 121 is removedby the resist stripping technique. Accordingly, the line-and-spaceshaped word lines WL which are arranged at a predetermined interval inthe Y direction are formed to extend in the X direction between a pairof the selection gate lines SGL which extend in the X direction.

In the fifth embodiment, the effects as those of the fourth embodimentcan also be obtained.

In addition, although a pattern forming method for the selection gatelines SGL and the word lines WL in a NAND type flash memory is describedin the above embodiments, the above embodiment may be applied to a caseof forming a fine pattern and a pattern of which the size is larger thanthat of the fine pattern.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

1. A pattern forming method comprising: forming a first core film abovea processing target; forming an opening pattern in a predetermined areaof the first core film; conformally forming a mask film above theprocessing target above which the opening pattern is formed; performingetch-back of the mask film until an upper surface of the first core filmis exposed so that the mask film remains on a side surface of the firstcore film; forming line-and-space shaped first core patterns that aremade of the first core film and in which widths of line patterns havesmaller than a width of the mask film remaining on the side surface ofthe first core film in an area other than an area forming the openingpattern; conformally forming a first sidewall film above the processingtarget above which the mask film and the first core patterns are formed;performing etch-back of the first sidewall film so that upper surfacesof the first core patterns are exposed; forming first sidewall patternswhich are made of the first sidewall film above the processing target byremoving the first core patterns; and patterning the processing targetby using the mask film and the first sidewall patterns.
 2. The patternforming method according to claim 1, wherein the processing target is anobject where a plurality of memory units are formed in a semiconductorsubstrate including element isolation insulating films which extend in afirst direction and are arranged at a predetermined interval in a seconddirection perpendicular to the first direction, each memory unitincluding a gate insulating film, a floating gate electrode film, aninter-electrode insulating film and a control gate electrode film whichare stacked in order on the semiconductor substrate, in the forming ofthe opening pattern, the opening pattern is formed in such a manner thatan area including a pair of selection gate lines, extending in thesecond direction and facing each other, of the memory units that areadjacent to each other in the first direction is open, and in theforming of the first core patterns, the first core patterns are formedin an area including line-and-space shaped word lines formed between apair of the selection gate lines that are disposed at respective bothends of the memory unit in the first direction.
 3. The pattern formingmethod according to claim 2, wherein in the performing of etch-back ofthe first sidewall film, the mask film and the first sidewall filmsurrounding the mask film are formed at a formation position of theselection gate line, and in the forming of the first sidewall patterns,the first sidewall patterns are formed at a formation position of theword lines.
 4. The pattern forming method according to claim 2, whereinin the forming of the mask film, a thickness of the mask film on theside surface of the first core film is set to be a value obtained bysubtracting a width of a predetermined number of the word lines from awidth of the selection gate line.
 5. The pattern forming methodaccording to claim 1, wherein in the forming of the first core film, asecond core film of a material different from that of the first corefilm is formed between the processing target and the first core film,and the patterning the processing target includes, forming second corepatterns by etching the second core film using the mask film with thefirst sidewall film surrounding the mask film, and the first sidewallpatterns, sliming the second core patterns, conformally forming a secondsidewall film above the processing target above which the slimmed secondcore patterns are formed, performing etch-back of the second sidewallfilm so that upper surfaces of the second core patterns are exposed,forming second sidewall patterns made of the second sidewall film byremoving the second core patterns other than the second core patterncorresponding to a formation position of the mask film, and etching theprocessing target using the unremoved second core pattern and the secondsidewall patterns.
 6. The pattern forming method according to claim 5,wherein in the sliming of the second core patterns, the second corepatterns are slimmed so that widths thereof become substantially halfwidths of the first core patterns.
 7. The pattern forming methodaccording to claim 6, wherein in the forming of the first core patterns,after processing the first core film to line-and-space shaped patternshaving first widths, the first core films are slimmed until widths ofthe patterns become substantially half the first widths.
 8. The patternforming method according to claim 5, wherein the processing target is anobject where a plurality of memory units are formed in a semiconductorsubstrate including element isolation insulating films which extend in afirst direction and are arranged at a predetermined interval in a seconddirection perpendicular to the first direction, each memory unitincluding a gate insulating film, a floating gate electrode film, aninter-electrode insulating film and a control gate electrode film whichare stacked in order on the semiconductor substrate, in the forming ofthe opening pattern, the opening pattern is formed in such a manner thatan area including a pair of selection gate lines, extending in thesecond direction and facing each other, of the memory units that areadjacent to each other in the first direction is open, and in theforming of the first core patterns, the first core patterns are formedin an area including line-and-space shaped word lines formed between apair of the selection gate lines that are disposed at respective bothends of the memory unit in the first direction.
 9. The pattern formingmethod according to claim 8, wherein in the performing of etch-back ofthe second sidewall film, the second core pattern corresponding to theformation position of the mask film and the second sidewall filmsurrounding the second core pattern are formed at a formation positionof the selection gate line, and in the forming of the second sidewallpatterns, the second sidewall patterns are formed at a formationposition of the word lines.
 10. A pattern forming method comprising:forming a first mask film above a processing target; forming an openingpattern in a predetermined area of the first mask film; conformallyforming a second mask film of a material different from that of thefirst mask film above the processing target above which the openingpattern is formed; performing etch-back of the second mask films untilan upper surface of the first mask film is exposed so that the secondmask film remains on a side surface of the first mask film; forming athird mask film formed of the same material as the first mask film abovethe processing target after the second mask film remains on the sidesurface of the first mask film; planarizing an upper surface of thethird mask film by using the second mask film as a stopper; formingline-and-space-shaped first core patterns, that are made of a first corefilm and in which widths of line patterns have smaller than a width ofthe second mask film remaining on the side surface of the first maskfilm, in an area other than an area forming the opening pattern of theprocessing target after the third mask film is planarized; conformallyforming a first sidewall film above the processing target above whichthe first core patterns are formed; performing etch-back of the firstsidewall film so that upper surfaces of the first core patterns areexposed; forming first sidewall patterns made of the first sidewall filmby removing the first core patterns; forming mask patterns by etchingthe first mask film and the third mask film using the first sidewallpatterns; and patterning the processing target by using the second maskfilm and the mask patterns as masks.
 11. The pattern forming methodaccording to claim 10, wherein in the forming of the first corepatterns, after processing the first core film to line-and-space shapedpatterns having first widths, the first core films are slimmed untilwidths of the patterns become substantially half the first widths. 12.The pattern forming method according to claim 10, wherein in the formingof the first core patterns, a second core film of a material differentfrom that of the first core film is formed between the third mask filmand the first core film, and the forming the mask patterns includes,forming second core patterns by etching the second core film using thefirst sidewall patterns, sliming the second core patterns, conformallyforming a second sidewall film above the processing target above whichthe slimmed second core patterns are formed, performing etch-back of thesecond sidewall film so that upper surfaces of the second core patternsare exposed, forming second sidewall patterns made of the secondsidewall film by removing the second core patterns, and etching thefirst mask film and the third mask film using the second sidewallpatterns.
 13. The pattern forming method according to claim 12, whereinthe processing target is an object where a plurality of memory units areformed in a semiconductor substrate including element isolationinsulating films which extend in a first direction and are arranged at apredetermined interval in a second direction perpendicular to the firstdirection, each memory unit including a gate insulating film, a floatinggate electrode film, an inter-electrode insulating film and a controlgate electrode film which are stacked in order on the semiconductorsubstrate, in the forming of the opening pattern, the opening pattern isformed in such a manner that an area including a pair of selection gatelines, extending in the second direction and facing each other, of thememory units that are adjacent to each other in the first direction isopen, and in the forming of the first core patterns, the first corepatterns are formed in an area including line-and-space shaped wordlines formed between a pair of the selection gate lines that aredisposed at respective both ends of the memory unit in the firstdirection.
 14. The pattern forming method according to claim 13, whereinin the performing of etch-back of the second mask film, the etch-back ofthe second mask film is performed so that the second mask film remainsat a formation position of the selection gate line, and in the formingof the second sidewall patterns, the second sidewall patterns are formedat a formation position of the word lines.
 15. A pattern forming methodcomprising: forming a core film above a processing target; forming corepatterns including line-and-space shaped patterns by processing the corefilm; conformally forming a first sidewall film above the processingtarget above which the core patterns are formed; performing etch-back ofthe first sidewall film so that upper surfaces of the core patterns areexposed; forming first sidewall patterns made of the first sidewall filmabove the processing target by removing the core patterns; conformallyforming a mask film above the processing target above which the firstsidewall patterns are formed, so as to embed portions between theadjacent first sidewall patterns formed in an area forming theline-and-space shaped patterns; performing etch-back of the mask filmuntil upper surfaces of the first sidewall patterns are exposed so thatthe mask film formed on side surfaces of the first sidewall patterns ofend portions without being embedded between the adjacent first sidewallpatterns remains; forming mask patterns that include line-and-spaceshaped first patterns corresponding to the mask film embedded betweenthe adjacent first sidewall patterns and a second pattern correspondingto the mask film remaining on the side surfaces of the first sidewallpatterns of the end portions, by removing the first sidewall patterns;and patterning the processing target by using the mask patterns.
 16. Thepattern forming method according to claim 15, wherein in the forming ofthe core patterns, after processing the core film so as to includeline-and-space shaped patterns having first widths, the core films areslimmed until the widths of the patterns become substantially half thefirst widths.
 17. The pattern forming method according to claim 15,wherein the patterning the processing target includes, sliming the maskpatterns until the mask patterns have widths substantially half widthsof the first patterns, conformally forming a second sidewall film abovethe processing target above which the slimmed mask patterns are formed,performing etch-back of the second sidewall film so that upper surfacesof the slimmed mask patterns are exposed, forming second sidewallpatterns made of the second sidewall film formed in an area forming theline-and-space shaped patterns by selectively removing the firstpatterns among the exposed mask patterns so that the second patternsurrounded by the second sidewall film is not removed, and etching theprocessing target using the second sidewall patterns and the secondpattern surrounded by the second sidewall film as masks.
 18. The patternforming method according to claim 15, wherein the processing target isan object where a plurality of memory units are formed in asemiconductor substrate including element isolation insulating filmswhich extend in a first direction and are arranged at a predeterminedinterval in a second direction perpendicular to the first direction,each memory unit including a gate insulating film, a floating gateelectrode film, an inter-electrode insulating film and a control gateelectrode film that are stacked in order on the semiconductor substrate,in the forming of the core patterns, the line-and-space shaped patternsare formed in an area including line-and-space shaped word lines formedbetween a pair of the selection gate lines which are disposed atrespective both ends of the memory unit in the first direction andextend in the second direction.
 19. The pattern forming method accordingto claim 18, wherein in the forming of the core patterns, the corepatterns further including a pattern covering an area including a pairof the selection gate lines, facing each other, of the memory units thatare adjacent to each other in the first direction are formed.
 20. Thepattern forming method according to claim 18, wherein in the forming ofthe mask patterns, the first patterns are formed at a formation positionof the word lines, and the second pattern is formed at a formationposition of the selection gate line.